The present invention relates to the field of monovinyl aromatic compound purification and polymerization, and more particularly discloses processes and apparatus for reduction the of phenylacetylene contaminants in crude styrene feedstock.
Of all the thermoplastics manufactured today, probably the most versatile and most widely utilized class of materials is polymerized monovinyl aromatic compounds, such as polystyrene, polymerized alpha-methyl styrene, and polymers of ring-substituted styrenes.
Some of the most common uses of these compounds (often referred to collectively as xe2x80x9cstyrenesxe2x80x9d or xe2x80x9cpolystyrenesxe2x80x9d) are for food and beverage containers, food wrap, and children""s toys. One disadvantage associated with such uses of polystyrene is the residual monomer and other contaminants in the polymer which may contribute to off-taste, odor, off-color and other adulteration or degradation of the polymer.
A particularly offensive contaminant associated with such undesirable properties in polystyrene is unreacted vinyl aromatic monomer, usually styrene monomer. One of the causes of unreacted monomer is directly related to the presence of phenylacetylene in tile styrene feedstock going into the polymerization reactor system.
In the manufacture of monovinyl aromatic polymer compounds and more particularly in the manufacture of polystyrene, benzene is reacted with ethylene to form ethylbenzene. This molecular compound is then dehydrogenated in an EB Dehydro unit to form styrene monomer. The styrene monomer is then polymerized, usually in the presence of a polymerization initiator or catalyst, to form the final polystyrene raw material.
Unfortunately, phenylacetylene, one of the undesirable side products of the EB Dehydro unit, is formed when ethylbenzene is dehydrogenated one step too far. Consequently, the product stream from the Dehydro unit contains styrene, ethylbenzene, and traces of phenylacetylene. The ethylbenzene easily is removed by conventional processes, such as distillation, leaving styrene monomer and phenylacetylene. The removal of phenylacetylene cannot be accomplished by distillation and has heretofore been difficult and costly.
The presence of phenylacetylene in styrene monomer has undesirable consequences regardless of whether the method of polymerization utilized comprises anionic, or free-radical polymerization. During anionic polymerization, phenylacetylene which is slightly acidic, consumes a stoichiometric amount of catalyst, such as butyllithium, wherein one molecule of butyllithium is removed from the polymerization process by each molecule of phenylacetylene. This loss of catalyst can be costly and causes the concentration of catalyst to be difficult to control. This, in turn, causes the molecular weight of the polystyrene to be difficult to control and can result in an increase in the concentration of low molecular weight polymer and even unreacted styrene in the polystyrene.
During free-redical polymerization, the presence of phenylatetylene can have detrimental effects on chain length and polymerization rate, because it is a poor chain transfer agent. Consequently, in the manufacture of polystyrene beads, which are used to make expanded or xe2x80x9cfoamedxe2x80x9d polystyrene, significant amounts of residual styrene are left in the bead.
Styrene is a suspected carcinogen and creates undesirable taste, odor, and health hazards, when present in small amounts in polystyrene.
Thus, the presence of phenylacetylene in styrene monomer has adverse effects on cost, control of the polymerization process, and purity of the resulting polystyrene. The presence of phenylacetylene in polystyrene also results in olefinic bonds in the backbone of the polymer which can increase cross-linking and cause more rapid oxidation of the polymer, both of which degrade the polymer.
In free-radical polymerization of styrene, as the concentration of styrene goes down during the polymerization process, the relative concentration of phenylacetylene naturally increases, and, since phenylacetylene acts as a polymerization inhibitor, the polymerization process is undesirably affected.
Catalytic attempts at reducing the phenylacetylene levels in styrene monomer streams have involved the injection of high levels of hydrogen gas into the monomer in an attempt to reduce the phenylacetylene to styrene. Any hydrogen added into the stream in stoichiometric excess of the phenylacetylene present also resulted in a significant conversion of styrene back to ethylbenzene, causing a lower styrene concentration and a lower conversion rate. Significant reductions in phenylacetylene were achieved only at the expense of styrene conversion to EB and resultant loss of styrene production.
The present invention discloses processes and apparatus which achieve a reduction in the phenylacetylene levels of monovinyl aromatic monomer feedstock in polymerization systems by the use of either a two-bed reactor to replace the single-bed reactors, or a pair of catalyst reactors, each bed or reactor having injection means for injecting hydrogen gas or a mixture of hydrogen gas and a diluent into the monomer reaction stream to reduce phenylacetylene to styrene.